Ophthalmic lens



March 6, 1951 G. B. woo'rTl-:N

OPHTHALMIC LENS 4 Sheets-Sheet 1 Filed Nov. 4, 1949 Q emma Jwerzl'or 650/265 3, WoorE/v 35: Mzgg; Q.

QMIOI QQQ March 6,

Filed Nov.

ZZHMM 17150 1951 G. B. wooTTEN OPHTHALMIC LENS kiff/@EME L/NE 4 Sheets-Sheet 2 ENS THICK/V555 i i W 0./8 MM 0.1/ MM SAME Ffm/v7 sam-'Aas awww/R5 ons can/zeri SER/fs March 6, 1951 G, B, WQTTEN 2,543,949

OPHTHALMIC LENS Filed Nov. 4, 1949 4 Sheets-Sheet 3 G. B. WOOTTEN OPI-ITHALMIC LENS March 6, 1951 4 Sheets-Sheet 4 Filed Ney! 4, 1949 Patented Mar. 6, 1951 UNITED STATES PATENT OFFICE OPHTHALMIC LENS George Bates Wootten, Sudbury, Ontario, Canada Application November 4, 1949, Serial No. 125,532

' 13 Claims.

This invention relates to an ophthalmic lens of novel type herein referred to as a lens of minimal deviation which-is constructed in accordance with the principle of minimum deviation but wherein such may embody zero deviation.

This application forms a continuation in part of my application Serial No. 721,524 filed January l1, 1947, now abandoned.

According to the invention, if the problem of refraction through any zone of a lens is reduced to its simplest terms it appears that the ultimate in lens form must be based upon a criterion of the best disposition .of each zone in relation to' the refracted rays. As the eye rotates to regard objects through various zones of a lens, it is concerned at any one time with only a small pencil of light rays hereinafter referred to as sight rays. For purposes of definition the term principal sight ray as used herein refers to a particular sight ray coming from the point of regard and which is directed by thelens to the eye along its visual axis. By disposing each elemental prism or lens zone of a lens in a position .of minimum deviation, the various aberrations accounted for in ophthalmic lenses are reduced to minimal amounts. The term minimum deviation as used herein embodies the generally accepted meaning as applied to a prism which is so placed that incident and emergent rays make equal' angles with the normals at their respective surfaces with a minimum degree of deviation.

Having regard to a criterion of minimum deviation in all zones of a lens, it is notable that a special case would involve a lens of zero deviation wherein the lens form would comprise concentric front and rear surfaces. I have found that .a lens of this particular type approaches a practical form when of a power of about minus 1 dioptre. I call such a lens a reference lens since in deriving a series lof lenses of the same general form for other powers but adhering Vto the criterion of minimum deviation, dimensions of a specific lens are derived from the reference lens form.

'Ihe lens of the invention may therefore embody minimum deviation of a sight ray or zero deviation of such ray, wherefore to vavoid confusion, `and having regard to the general principles involved herein I term such embodiment broadly a minimal deviation lens since such sight ray may suffer either minimum or zero .deviation according to lens form taught hereinafter in more detail. Therefore it will be understood that my criterion vor principle ,stated as minimal deviation may embody both of these noted prior principles as applied to prisms.

It has been found that in selecting a reference lens which will be Aclose to minus l power, the rear surface vertex distance from the centre of ocular rotation is set as equal to the radius of the rear surface where the front surface is concentric therewith. These various factors will determine the lens form of the reference lens which will operate at zero deviation. A11 other lenses of a series derived from a reference lens and known herein `as "related lenses are modified only in accordance with the criterion of minimum deviation but preferably in the following manner: The front surface curvature is set as constant for all lens powers above a value corresponding to the minimum practical lens thickness to be ex,- plained hereinafter in more detail. Above this reference power the lens thickness is varied in accordance with the values imposed by the rear surface curvature in turn determined by a minimum deviation condition for sight rays. Below this power value referred to, the thickness is kept constant and the front and rear surface radii are modified as hereinafter more fully illustrated in detail.

`At once a distinct advantage accrues in that it is possible to employ a single lens blank form for a number of lenses in any one series extending from approximately minus 4.5 dioptres up to and past plus 10 dioptres if desired, by reason of a single value of front surface curvature. This provides a distinctive saving to a lens manufacturer.

With the above and other objects hereinafter set forth, the invention generally comprises an ophthalmic lens having affinishe'd front surface and a finished rear surface defining a lens body in which the front and rear surfaces of any elementary zone thereof make equal angles respecn tively with incident and emergent components of a sight ray passing therethrough to obtain minimal deviation of the said sight ray. The rear surface of the lens body is disposed a predetermined distance from .the centre of ocular rotation such that the emergent component of the sight ray of minimal deviation intersects the principal axis of the lens within a section of the said axis defined by the points of intersection of.

the radii of .curvature `of the surfaces thereof. In the `special .case .of the ,reference lens the deviation .condition will be zero for all zones and the .concentric point kof the lens surfaces will be at the centre of ocular rotation.

Other objects of the invention will be appre- 3 ciated by a study of the following specification taken in conjunction with the accompanying drawings.

In the drawings,

Figure 1 is a schematic illustration representing the derivation of related lenses of a lens series from a referencelens of chosen form.

Figure 2 is a chart embodying curves of front surface curvature, lens thickness, and emergent ray departure from the centre of ocular rotation on a lens power base for lenses derived from a reference lens of minus 1 dioptre having its rear vertex spaced 25.5 mm. from the centre of ocular rotation and having a front surface of 19 dioptres down to a minus 4.5 dioptre lens.

Figure 3 is a diagrammatic view of a reference lens having zero deviation.

Figure 4 is a diagrammatic view of a convex lens section having minimum deviation characteristics according to the invention.

Figure 5 is a similar diagram for a concave lens.

Figure 6 is a diagram illustrating emergent ray departure for various field angles.

Referring now in detail to the drawings and particularly to Figure 1 which is representative only, it will be observed reference lenses I0, II and I2 are shown of different powers and which have front and rear surfaces concentric with a point I3, I4 and I5 respectively, which according to the invention is located at the centre of ocular rotation. From the drawing it will be apparent that as reference lenses of greater minus power are chosen they will become thicker and the thickness will be reduced to zero at zero power. For purposes of illustration herein, it is assumed that a minus 1 doptre lens of front and rear concentric surfaces will have a form which is suitable assuming a rear surface vertex distance of 25.5 mm. The thickness will be found to be 2 mm.

Having decided uponthe general lens form, a lens series having minimal deviation may be derived and any one related lens determined having regard to a criterion of minimum -deviation. At the minus l dioptre power the front and rear surfaces will be concentric and in any elementary zone thereof will be parallel as illustrated by lens sections I6. As the lens form is modified according to the criterion of minimum deviation toward the plus end of the scale, it is modified to the characteristics represented by the lens form I1 wherein each zonal portion of the lens accommodates a sight ray in the manner indicated at I8. In the direction of the more minus powers from the reference lens, the lens body becomes thicker at its ends as indicated by the lens form I9 having zonal sections of the type indicated at 20.

Assuming specifications for a reference lens form such that the front surface curvature is 19 dioptres and the rear surface vertex distance is 25.5 mm. from the centre of ocular rotation, then the lens form throughout a series is predictable from Figure 2 which is, of course, representative only of actual curves. According to the invention, front surface curvature preferably is kept at a constant value over a substantial range of powers extending in this case from minus 4.5 dioptres to plus dioptres. Below the value of minus 4.5 dioptres the lens thickness at the vertex would become impractical and therefore, it is desirable to set the vertex thickness as a constant factor down to minus 10 dioptres. Representative lens forms of related lenses are Shown 0n EL lens base in this figure, the lowermost set of curves illustrating the departure of the intersecting point of an emergent sight ray of minimum deviation with the principal axis from the centre of ocular rotation. According to the invention all lenses of any one series will have the same rear surface vertex distance measured along the principal axis to the centre of ocular rotation. These curves will be referred to in more detail hereinafter.

Table I following sets forth values of vertex thickness, front surface power, and rear surface distance from the centre of ocular rotation for a complete lens series for each power from plus 10 to minus l0 dioptres, so that the scope of lens form herein set out will be more readily appreciated by skilled persons.

Table I V Sugacc Lens Front Sur- Rear Surl-m* cl ex s' Power face Power facc Power g-lw lar Rotation Doptres Dioptres Doptres Mm. Mm. 10. 00 +17. G20 27. 744 0. 6 25. 5 8. 00 +18. 125 26. 256 0. 6 25. 5 6. 00 +18. 640 24. 791 0. 6 25. 5 4. 5() +19. 025 23. 670 0. 6 25. 5 2. 00 +19. 025 21. 413 1. 6 25. 5 1. 00 +19. 025 20. 518 2. 0 25. 5 +2. 00 +19. 025 17. 818 3. 2 25. 5 +4. 00 +19. O25 16. 026 4. I] 25. 5 +7. 03 +19. 025 13. 347 5. 2 25. 5 +10. O0 +19. 025 10. 680 6. 4 25. 5

Having reviewed the derivation of lens form generally from a reference lens it is now pertinent to detail the factors involved in such derivation. It seems apparent that certain factors have been overlooked in prior proposals in the reduction of astigmatic aberrations in ophthalmic lenses, namely, facts pertaining to the principle of minimum deviation as it may be applied to ophthalmic lenses and set out in more detail hereinafter.

In present proposals account is taken of the lfact that only a very small pencil of slight rays falls directly along the visual axis at any time and that, therefore, this pencil of sight rays may be regarded for purposes of calculation, as being a single ray termed herein a principal ray. It is a general law of optics that when any quantity is passing through its minimum value, a small change in the variable concerned produces very little effect upon the magnitude of the quantity itself. For example, deviation of rays through a prism depends upon the path of the rays but when the prism is in the position of minimum deviation any small change in the path of the rays produces but little change in the magnitude of the deviation.

Therefore, when considering rays passing through a prism by paths near to that of minimum deviation, it will be apparent that the deviation which each undergoes is practically the same and very nearly equal to the minimum value. Thus it is consistent with the general law set forth to consider that all rays of a small pencil will be deviated to an approximately equal extent where the principal ray passes along a path of minimum deviation. While this application of the law applies to a plain prism it will be appreciated that small pencils of sight rays may be refracted substantially at the angle of minimum deviation for each zone of the lens when suitable lens form is employed. The related lens forms of this invention are based on this concept.

Now referring to the drawings and particularly Figure 3 a reference lens 2| is illustrated having a front surface 22 and a rear surface 23 with radii R1 and Rz respectively scribed concentrically from the common point O located on the principal axis 24. The point O is, according to the invention, positioned at the centre of ocular rotation. A principal ray 25 is shown passing through the lens by a. path of minimal deviation being in this case zero deviation, to meet the principal axis 24 at the point O. It will be appreciated that in consideration of ophthalmic lenses being the only type considered herein, a discussion of any other rays is irrelevant. The concentric lens in Figure 3 is of the reference lens type. This lens form has uniform thickness throughout. For an assumed distance of 25.5 mm. (preferred herein) between the point O and the rear surface vertex 26, a reference lens of minus power is exemplified. For purposes of illustration, a lens of minus 1.00 D proportions is illustrated.

Related lenses of lower minus refractive power according to these proposals present the uncommon case of concave-convex minus lenses wherein the centre thickness is greater than the edge thickness. A lens of higher minus power with substantially reduced edge thickness as compared with conventional lens forms would therefore be feasible, Vgiving a feature greatly appreciated by myopes. Obviously lenses designed to be used for safety purposes may be much thicker than the thickness allowance of 0.5 dioptre in the refer-` ence lens suggested herein for the more common lens prescription. Safety lenses may also be placed farther from the eyes. These factors would Vaffect the reference lens to give `a somewhat shallower form and a front surface curvature of longer radius. It is, therefore, feasible to have different reference lenses according to the general form desired, as follows:

Table II l Radius .Radius Rear Lens of Front of Rear Thickness Surface Power Surface Surface Allowance Di/gge Mm Mm. Mm. 1. O 27. 5 25. 5 (2 mm. 0.5 D 25. 5 -1. 6 28. 6 25.4 (3.2 mm.) 0.75 D 25. 4 -l. 6 29. 9 26. 3 (3.6 mm 0.75 D l26. 3

l tions for lenses of different powers.

According to the invention it is necessary to keep front surface radii of curvature within certain limits. For a lens of minus 1.00 D power these limits are between 26.4 mm. and 29.0 mm. Translated into terms of front surface plus power these would read 19.8 D and 18.0 D'respectively for a minus 1.00 D lens. Table III shows a variety of useful front surface plus powers for each of a series of specimen lenses over a range of lens powers most frequently used in practice.

location of this xed point m divides the line The term lenses of the minimal deviation type is thus considered to embody both reference and related lenses, the most practical of which fall within the scope of Table III.

-' For. a discussion of the theory of the function of related lenses reference is now made to Figlires 4 and 5. A ray 2l is shown being transmitted by the lens in a position of minimum deviation. That part of the ray within the body of the lens may have a theoretical projection M intersecting the principal axis at a point m. It has been found by mathematical analysis that, for lenses bounded by one concave and one convex surface, the point m is common to all such aforesaid projections from all zones of the lens. The

(joining the centres of surface curvatures) sec'- tion C102, internally in the ratio wherein c is the segment Cim, b is the segment mCz, and R1 and R2 are the radii of front and rear surface curvatures respectively.

A ray of minimal deviation in this case more precisely one of minimum deviation, upon emerv gence from the lens body, is refracted to cross the axis of the lens at a point e which is away from the point m inthe direction of C1 but never so far as C1. If the distance from Cato a is represented .by af then the mathematical relationships which exist pertaining to the location of point e are:

Ri-I- R2 wherein b is positive for related lenses of more negative power than the reference lens and b is negative for related lenses of more positive power than the reference lens, and

Ib sin m sin z wherein I represents the index of refraction of and for a lens more positive than the reference lens, angle` z=m+` (A-B) (6) In either case if the angle at m is given it is possible to find B from the relation Thus with the given angle m and the angles A and B, the corresponding angle a can be evaluated for use in evaluating "a.

In the formula for a the angle z is dependent upon the angle m and can be evaluated for any value of the latter. Therefore the value a is dependent entirely upon the one variable m, being the angle that any ray of minimum deviation within the lens body makes with the principal axis of the lens.

ua is obviously a Variable. However, since (A-B) the amount by which angle s differs from the angle m, being the angle of minimum deviation, is small compared with angle m, thenrthe fraction sin m sin z in (4) above, will not differ greatly from the value unity, nor vary greatly throughout the range of angle m. Therefore the value of 0., or the position of z will not vary greatly for the various emergent rays of minimum deviation passed through various zones of the lens.

Now referring to Figure 2, the magnitude of the zone or range Zo for lenses of various powers` and the departure along the principal axis of emergent rays of minimal deviation from the centre of ocular rotation at field angles of 45, 25 and 5, is shown for a lens series derived from a minus 1 reference lens having 0.5 D thicknesszone Z0 on the principal axis which includes all the points z up to a field angle of 45 works out to 0.075 mm. It will be clear that the magnitude of this range Z0, in comparison with the range of accuracy in fitting a lens before the eye, is exceedingly small and therefore the extent of the range Z0 may be assumed to be a point which is xed for all practical purposes. Figure 2 illustrates the manner in which the lens series has been designed to place the range Zo within which rays of minimal deviation cross the axis so that it embraces the assumed position of the centre of ocular rotation.

The range Z0, small though it is, is still not indicative of the true performance of the lenses of the invention. All rays of minimal deviation approaching the axis at an angle pass closer to the centre of ocular rotation than is indicated by the distance Zo measured along the axis.

Thus in Figure 6 a principal axis 30 has located on it the centres of the arcs of the front and rear surfaces as at C1 and C2. A number of' which, for all practical purposes, coincides with` axis to intersect at the points al, z2 and The apparent departure insofar as the eye is concerned is represented by a line drawn at right l angles to each of the raysand extending to in-v tersect the centre of ocular` rotation O as indicated by letters yl and g3.

Apparent departure y=(zO) sin a where 2O represents the distance between the point of intersection of any emergent ray pass-l ing to intersect the principal axis in accordance with minimal deviation and the centre of ocular..

rotation O.

Since the sine function is always less than unity, it will be observed that the value of y will always be less than that of 20. It will be apparent from Figure 2 that at some field angle between 25 and 45. the departure curve will lie substantially along the zero line and it is for this reason that; ray 2 in Figure 6 is shown passingv through the centre of ocular rotation. The principal departure (zO) and the apparent departure y in ythis instance will have value zero. For eld angles less than substantially this value, the ray will make a small angle e with the axis, and since.

the sine values of small angles decrease to zero with the angles, the departures y from the ocular centre will likewise become negligibly small for rays of minimal deviation from the zones of the' lens near the vertex'.

Thus for the minus 6 lens above discussed, in-

stead of considering the extent of the zone z asl a value equivalent to almost 0.1 mm. along the iprincipal axis, this value' would be more prac' tically represented by a figure of 0.031 mm. taking into account the sine relation above discussed.

rotation.

It will be apparent that a lens of the invention may be defined clearly for all practical purposes I. as having a front and rear surface formed in accordance with a criterion of minimal deviation to direct a sight ray passing through any zone of the lens bya path of minimal deviation towavrdthe principal axis to intersect the latter at a point the assumed centre `of ocular rotation of the wearer of the lens. It will be clear from an examination of the lens thickness curve in Figure 2 that the rate of change of thickness with change in dioptric power as one examines the lenses throughout a series is a constant. It will be seen in this diagram that the lens thickness varies in a straight-line relation from minus 4.5 D to plus 10 D, the line of change of thickness being on an apparent slope with respect to the line of lens powers. In considering the final lens forms illustrated in Figure 2 such relation holds true fromminus 4.5 dioptres to plus 10 dioptres and this portion of the diagram may be consideredra series.

A second determinate factor to lens thickness comes into play for more minus powers than minus 4.5 in the series shown, namely, that i of the minimum thickness necessary for desirable strengthl characteristics. Nevertheless, the theoretical lens is rst determined then the necessary thickness added for practical requirements and then the front surface radius vadjusted to obtain I thedesired effective lens power. In any case, in these morefminus powers, as will be evident in. Vthe front surface curvature diagram of Figure 2,`

the change of front surface curvature is constant fior this reason` deviation es previously dieouseeofl, may embody.

fp reoiated heine `that @se meiden? se@ emift l components of `the vsieht ray make .-eettlel angles 40 @pending-Turion lens-form. :Inanypase tljre y n irriaggriim? 11n this invention it 111be.appreciatecffpmgtne l curvature will be identical with tljiatj,r in a corresonA ing lens'of Spherical power only.

1sV a'fUSed Segment may be placed of the lens symmetrically in relation ht'raytra'nsmitted through its centre. A 7 s of this character has an advantage o; improxaed lens"lpos'tuie`leading tofurther ads. varftages'fadily appreciated by skilled persons. Will be readily $59- greater" protection andunimpeded eld of nafofded `by"protectiveI goggles and wsung u5.5 constructed according to these proposals Se' 'te' otite 'eoeefeot It Ais inten ed thatV the present disclosure should in the. thiokneee of. e eeneietei vette throughout e eurobet .o f. no

In oieniee this inveetiohl parent. to .Skilled eereens'thett in as the form varies @Revelli g'v rrornfeitherler'i of v one series .tls

the Series towers the tete' nee leesfthe eee-trs of iront enti teer Suriaoeeurvetures wilfb `1o- Gated .dieser toeether on th no 'tloefoonstriieci in any-limiting Sense'other than they eetl-lement e t e tefelen that indicated py the scope of' theronqwing emergent sieht ve .Si ke .al1-titties between the e l eleove d soueeed, Within e es the Yfotte is: ee tew @the r alone theefxis .this Zone diminishes turn beeomes Poi-et t. wiuithe eerittee efeur'vature et theretere oe len's' This will be evident from Figure 1. v1t will be clear, titer etereg that the emergent Atette do' not Strike the prineioegl ets-s outside Ithe*leeetit of the 25 centres of uri/attire, ithat is to ,say,` tl1'e .section of the oriental exis between these '-eenttee- YIt 1S A at .den Hng this invention, that the point of .intel-Stetten of as emergent rey defined as -fbeing located between thefcentres frost ,teer .Snrteoe zottrveturesi: 'stehlensslaee embodying the meeni-ttethet Stich iooilt -rnay be c eident wi 4such centres when vthe latter themselvesfeeinedeetfeueh eS le the case -of -tliereference lens. The termfffminimal .elemeif what I claim as my invention is:

` 1. A related ophthalmic lens of a lens series d ened bly a power range extending from about minus 10 dioptres to plus 10 dioptres, wherein all :lenses of said Series have a rear surface vertex .distance frorlrithe Centre of ocular rotation of the wearer corresponding to a normal wearing distance of a `value between 24 and 27 millimeters .whichis constant for all lenses of said series, said series*Y including .a reference lens of a power of about minus 1 dioptre wherein thev rear surface radiusis equal to'said rear surface `lverteX disytace'and the frontsurface thereof is concentric withthe rear surface, the vertex thickness and rear surface curvature of each lens of said series down nto about minus 4.5 diptresu being formed to' provide minimal deviation of a siglt ray pass- ,ning through anyrzone of the lens,`the lenses f Vthe seriesdown'to about minus' dioptrs havine a constant'front surface curvature of a'radius greaterthan tledistance of the rear surfacever- 'ftfr'inmtbe centre of ocular rotation, ythe lenses of said'se es of more 4minus"power than about *rriinll'islf having f' constant'vertex thicllfness4 and liavigtboth frontand're'ar surface curvatures the meanings of either minimum deviation' @jor @ero deyiatior-i, or bot-h, the main mfvactoroto -vbe apbytwo condi-tions, canmensual conditionlofnie oentrie time .end a power of eboutminus 1 di- Ypower necessary -togiv'e .iiliedesired'overall.lens .-{ODre ittd 1.911%@ Siti@ STQS 0f Y@ 90W?? power-1in coniunction `:with-tbe giron-t sur-tacefglb) the rad ius necessany` which, .in .'oriilunetion .with tha-.front surface (directs sight ray from any zone of the lens by a oath of minimal deviation toward the centre of ocular rotation.

The lenses lof the present invention will have their edees ooeitioned Closer to uthe ieee than ii lenses now in general use and ,itfwill therefore .be 6 5 @1 `,I ijstsllb @haben en -G11,Sgtlays-strilee thepTTJ- cpnarenttliat conventional frames and' mountcipal axis of lthe lens Within e .departure zone ings will not apply. There is,=`h`\vr, a greater eitigned hetween 4the `centres vci. curvature of eld yof-vision available through the lens .and-the ont'andrear surfaces, .andwliich embraces fgenenal ,deeper -curved formiwill' olacei-tlifeslens the vassumed cantre ,of ocular .rotation ,of the margins closer-to.theclineofrontour of the head 70.1.ivearer said firent andrea surace ,curi/attirs avoiding sharnlv nroiectiner ede'es and materially nevariediin.apglaDIB-Wlih:Qt'lfl 0f reducing possible accidentsin use. irnum, deviation .atgreater rnnllSpOW-elsblit '-Wbena fsnhero-cylindereisrequired, othebylnwhen:botbiaeyafted-f'the VBISX thickness Ofhe drical curvature may be placed on the rear sur- 4tiene Tremaiuine.a constant Vtlllefpng substanface of the lens, In such a case the front surface 'tially the minimum practical thickness desired.

tion and of any chosen vertex power of a value between minus dioptres to plus 10 dioptres and comprising: a finished spherical convex front surface radius of about 27 millimeters, a rear surface spaced from said front surface a distance greater than 0.5 millimeter at the vertex and of' a radius of a` value between 18 and 50 millimeters and of lesser radius than the radius of the front surface for minus powers and which in conjunction with the radius of the front surface provides minimal deviation of any sight ray passing through any zone of the lens such that all such sight rays strike the principal axis of the lens within a departure zone positioned between the centres of curvature of the front and rear surfaces.

5. An ophthalmic lens designed in accordance with a criterion of minimal deviation of a sight ray passing through any zone thereof from a point of regard to intersect the principal axis of the lens substantially at the centre of ocular rotation and comprising: a nished spherical front surface of about plus 19 dioptres, and a rear surface and lens thickness which for a vertex lens power of plus 10 dioptres have values of about minus 11 dioptres and 6 millimeters respectively, and for a vertex lens power of minus 4.5 dioptres are about minus 24 dioptres and 0.6 millimeter respectively, and which vary uniformly at least between said powers.

6. An ophthalmic lens as claimed in claim 5, in which the lens thickness is about 0.6 millimeter regardless of lens power and the front sur-- face and rear surface for a vertex power of about minus 4.5 dioptres are about plus 19 dioptres and minus 24 dioptres respectively, and for a vertex power of about minus 10 dioptres are about plus 18 dioptres and minus 28 dioptres respectively.

'7. A related ophthalmic lens as claimed in claim 3 wherein the distance of the rear surface vertex of the lens to the assumed centre of ocular rotation is a value between 24 and 2'? millimeters and the lens thickness at the vertex is greater than 0.5 millimeter.

8. A related ophthalmic lens as` claimed in claim 3, wherein the radius of front surface curvature is in all cases greater than the distance from the rear surface vertex to the assumed centre of ocular rotation, the distance of the rear surface vertex of the lens to the asequal to the distance from the centre of ocular rotation to the rear surface vertex, and a vertex thickness of predetermined value, said related lens comprising: a finished spherical front surface of a radius not less than the front surface radius of said concentric reference lens, a thickness determined by the condition that the ratio of change in thickness to the change of effective dioptric power throughout the lens series is equal to a constant, andthe rear surface being of a radius which in conjunction with said front surface and lens thickness gives the said effective power, to define a lens body in which the front and rear surfaces of any elementary zone thereof make equal angles with incident and emergent components of a sight ray passing therethrough to obtain minimal deviation thereof.

11. A related ophthalmic lens as claimed in claim 10 wherein the lens thickness is determined by the further condition that it be of a dimension sufficient for desirable structural rigidity, the said dimension being in excess of the computed thickness, and wherein the said front surface radius is modified to a value which in conjunction with the rear surface radius and modified lens thickness gives the said effective power.

12. An ophthalmic lens of an effective lens power between minus 10 dioptres and plus 10 dioptres, comprising in combination, a lens body dened by a finished front surface and a finished rear surface wherein the front and rear surfaces of any elementary zone of said body make equal angles respectively with incident and emergent components of a sight ray" passing therethrough to obtain minimal deviation thereof, so that when Said lens body is disposed a predetermined normal wearing distance from the centre of`ocular rotation said emergent component of said sight ray intersects the principal axis of the lens immediate the centre of ocular rotation, the distance from the rear surface vertex at the saidprincipal axis to the centre of ocular rotasumed centre of ocular rotation is of, a value between 24 and 27 millimeters, and the lens thickness at the vertex is greater than 0.5 millimeter. 9. A related ophthalmic lens as claimed in claim 3, wherein the radius of front surface curvature is in all cases greater than the distance from the rear surface vertex to the assumed centre of ocular rotation, the distance of the rear surface vertex of the lensto the assumed centre of ocular rotation is of a value between 24 and 2'7 millimeters, and the lens thickness at the Vertex is greater than 0.5 millimeter, said rear surface havinga radius between 18 and 50 millimeters and of a lesser radius than the radins of the front surface for minus lens powers at least.

tion being of a value between 24 and 27 millimeters, the radius of curvature of the front surface being greater than the said distance.

13. An ophthalmic lens comprising in combination: a lens body defined by a finished front surface and a nished rear surface wherein the front and rear surfaces of any elementary zone of said body make equal angles respectively with incident and emergent components of a sight ray passing through said zone to obtain minimal deviation of the said sight ray so that when said lens body is disposed a predetermined normal wearing distance of a value selected from the range 24 to 27 millimeters from the rear surface vertex to the centre of ocular rotation, said emer- 10. A related ophthalmic lens of a lensseries gent component of said sight ray intersects the principal axisA of the lens at a point less than 0.5 millimeter from tbe centre of ocular rotation.

' GEORGE BATES WOOTTEN.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 2,021,778 Hammon Nov. 19, 1935 FOREIGN PATENTS Number Country Date -466,620 Great Britain June 1, 1937 

